photobug56 wrote: ↑Sun Oct 03, 2021 1:42 am
And from what I've seen and what you are saying, the wheels on a given axle are solidly attached to the axle so that the 2 wheels and axle normally spin as one. I'm wondering if that's just to save money or if there is another reason.
It's not to save money, the solid axles are actually essential to how a train steers itself. As someone else already explained, the wheels are conical. If a truckset is riding too far to the right side of the rail, the difference in diameter of the wheels combined with the fixed axle cause the train to steer back to the left.
TurningOfTheWheel wrote: ↑Sun Oct 03, 2021 12:12 am
The Wiki article on hunting oscillation has a useful animation to see what's going on. I can't explain the phenomenon very well, but as I understand it, at high enough speeds, slippage can occur and the two wheels on each axle can start spinning at different speeds. When this happens, there is an oscillation induced in the vertical axis—from a bird's-eye view, the axle will appear to turn back and forth. It's called a "hunting oscillation" because the wheels are "hunting" for some equilibrium where they are either both spinning at the same rate or agree on the direction they should be headed given the difference in their rotational speeds.
https://en.wikipedia.org/wiki/Hunting_oscillation
The wheels on an axle cannot spin at different speeds. Let's say that a train is pushed to the right side, by some kind of unevenness in the railroad. The steering action of the axles will steer the train back to the left side, but it continues moving left until it's steered back again to the right again. Compare it with a pendulum clock, once its moving, it keeps oscillating, although with trains it only happens if the train is moving fast enough.
To combat it, there are often dampers installed between the truck and the body of the train. That absorbs the energy of the side-to-side motion. Compare it with the shocks in a car: Without it, if you drive over a bump, the car would be really bouncy.
There is one exception though: The Talgo cars don't have rigid axles at all. Instead, the wheels steer like a car, by a linkage that steers the wheel dependent on the angle between two adjacent cars. That way, there is no hunting.